Wave guide attenuator



April 29, 1952 R. B. NELSON WAVE GUIDE ATTENUATOR Filed June 23, 1945 Fig.4.

Inventor: Richard 13. Nelson WWMMAM Hus Attorneg.

Patented Apr. 29, 1952 WAVE GUIDE ATTENUATOR Richard B. Nelson, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application June 23, 1943, Serial No. 491,889

My invention relates to ultra high frequency systems and more particularly to an attenuator for utilization therein. It has been appreciated for some time that ultra high frequency electromagnetic waves, often referred to as microwaves, may. be transmitted dielectrically through a guide of the hollow-pipe type which may be constructed of a conductive material and which contains a dielectric medium such as air or a gas through which the energy of the wave is transmitted. Of course, the frequency of the exciting electromagnetic waves must be greater than the critical minimum frequency established by the dimensions, principally the transverse dimensions, of the guide.

In accordance with my invention described hereinafter, I provide a new and improved attenuator for systems of this nature wherein accurately controllable and determinable amounts of energy may be transmitted upon energization or excitation of; the guide.

It; is an objectv of my invention to provide a new andimproved attenuator for ultra high frequency systems.

It. is another object of my invention to provide a new and improved attenuator for ultra high frequencysystems which comprise a dielectric guide of the hollow-pipe type and in which the effective resistance across the guide is altered gradually along the longitudinal dimension of the guide to avoid sudden transition of the guide characteristics and the introduction of undesired reflections.

It is a further-object of my invention to provide a new and improved attenuator for ultra high frequency transmission systems of the hollowpipe type in which substantial cancellation of waves. reflected by said attenuator is obtained.

It is a still further object of my invention to provide a new and improved attenuating: means for ultra high frequency waves transmitted through a guide of the hollow-pipe type which may be easily and accurately adjusted to obtain a desired degree of attenuation.

One of the features of myinvention is the employment of a strip of resistive material disposed longitudinally along a wall of a wave guide of the hollow-pipe type and having a portion projecting a desired distance into the guide to produde, attenuationof the high frequency energy being propagated along the guide without intro-- ducing' any undesired reflections of the high frequency' waves.

For a better understanding of my invention, reference may be had to the following descrip- 6 Claims; (Cl. 178-44) tion taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims. Figs. 1 and 2, which are, re.- spectively, a cross sectional vertical view and an end view, represent. one embodiment of my invention and apply to a wave guide of rectangular hollow-pipe type; Figs. 3 and i are, respectively, across sectional vertical view and an end view of another embodiment of my invention; and Fig. 5 is a cross sectional vertical view of a modification of the embodiment shown in Fig. 3.

Referring now to Figs. 1 and 2 jointly, my invention is there illustrated a applied to an ultra high. frequency system for controlling the transmission of energy through a dielectric wave guide of the hollow-pipe which comprises a conductive or metallic guide I0 which may have a rectangular cross section. For example, the top and bottom of the guide may be defined by conductive or metallic plates II and I2 and the side walls, by plates l3 and I l. The dielectric wave guide may be formed of any suitable conductive material, such as. copper or brass, and electromagnetic waves of the H01 type and of a frequency greater than the cutoff frequency of the guide may be propagated in a direction from left to right in Fig. 1, being supplied from any suitable source (not shown).

In accordance with my invention, the high frequency waves traveling-along the guide ID are attenuated by means of a strip I5 comprising a resistive, material I6, such as finely divided graphite, coated on a thin piece I! of dielectric material, preferably of the molded phenolic resin type having a width substantially equal to the horizontal width of the opening within the guide ID. The strip [5 i attached at its left-hand end to thebottom wall l2 of the wave guide by means of a screw I 8 and nut 19. The opposite, or righthand, end of strip l5 has a longitudinal slot 29 therein and a screw 2!, which extends through the slot 20 and has a head portion larger than the. width of slot 29, is used in conjunction with a nut 22 to hold this end of the strip I5 against the inner surface of the bottom wall l2.

The center portion of the strip I5 is fixed inwardly into the guide In from the inner surface of wall [2. in a uniform curve between the fixed ends. A screw 24 in threaded engagement with a hole in bottom wall 12 and having at its lower end a knurled head 25 and at its upper end a pair of washers 25, 2'! lying on opposite sides of the strip I5 around a slot 28, therein, provides mean for adjustably controlling thev amount. by which this central portion of strip l5 extends into guide I!) and, consequently, the attenutating effect of this strip of resistive material on the waves traveling through the guide. By adjustment of the knurled head 25, the strip l may be positioned at the center of the guide where the electric field is strongest to increase the amount of attenuation, or Withdrawn to the inner surface of the bottom wall l2 to decrease the amount of attenuation. Because of the gradual nature of the Change in impedance in the direction of propagation, there is no sudden transition in the impedance characteristic of guide [0 to give reflections of the high frequency waves traveling therethrough. Therefore, the-amount of reflection caused by my adjustable attenuator is low for a wide range of frequencies. Moreover, the amount of attenuation may be calibrated in terms of the setting of the adjusting screw 25.

In the modification of the invention as shown in Figs. 3 and 4, a fixed attenuator which gives even a smaller amount of refiection.than the attenuator of Figs. 1 and 2 is shown. In this form of the invention, a pair of strips of resistive materials 30, 3|, which may comprise a material similar to that of resistive strip l5, are shown as extending longitudinally along the opposite walls II and [2 of wave guide Ill. The strips 30, 3| are of equal length and of similar contour and the opposite ends of each strip are fixedly secured, as by gluing, to the inner surfaces of the opposite walls II, I2 of wave guide Hi. In order to secure substantial cancellation of any waves reflected by the strips 30, 3|, corresponding ends of these strips are displaced longitudinally in the wave guide In by a distance equal to a quarter wave length at the frequency of the energy being transmitted through the guide. Since the strips 30, 3| are of exactly similar shapes and are displaced a quarter wave length apart, it is apparent that any waves reflected by one of the strips, for example the strip 3 I are cancelled by a similar reflected wave from the strip 30 of substantially equal intensity and opposite phase.

In the modification of my invention shown in Fig. 5, the strips 32, 33, corresponding to the strips 30, 3| of Fig. 3, are made adjustable in position by means of adjusting screws 34, 35 of a construction similar to the adjusting screw 24 shown in Fig. 1. The points of engagement of the screws 34, 35 with their respective strips 32, 33 are displaced longitudinally along the guide at a distance equal to a quarter wave length at the frequency of the wave being propagated. Moreover, the screws 34 and 35 may be mechanically linked, as indicated by the dotted line 36, for uni-control so that the strips 32 and 33 are projected toward the center of the guides simultaneously and in exactly equal amounts. Since the points of engagement of the screws 34, 35 with the strips 32, 33 are displaced a quarter Wave length apart and corresponding ends of the strips are displaced longitudinally a distance equal to a quarter wave length, exactly similar contours of the two strips are obtained and substantial cancellation of reflected waves in the guide is secured.

It is, of course, apparent that, since there are no slots in the walls of the guide I0, a desired amount of attenuation may be obtained in the various forms of the invention illustrated without any leakage of power from the guide. Furthermore, the construction is such that easy and rapid adjustment of a desired amount of attenuation is obtained. Since the low reflection obtained is due primarily to the gradual change in impedance in the direction of propagation rather than to any tuning effect of the guide, a resultant minimum amount of reflection is produced over a wide range of operating frequencies. I

While I have shown and described a particular embodiment of my invention as applied to a dielectric wave guide of particular configuration and employing the various elements diagrammatically illustrated, it will be obvious to those skilled in the art that many modifications may be made and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An attenuator for high frequency energy in a dielectric wave guide system of the rectangular hollow-pipe type comprising a pair of strips of resistance material extending longitudinally along said guide, each of said strips having its ends attached to the inner surface of an opposite wall of said guide and its center portion extending inwardly from its respective wall a desired distance, corresponding ends of said strips being displaced longitudinally in said guide by a distance equal to a quarter wave length at the frequency of said energy.

2. An attenuator for high frequency energy in a dielectric wave guide system of the rectangular hollow-pipe type comprising a pair of strips of resistance material extending longitudinally along said guide, each of said strips having its ends attached to the inner surface of an opposite wall of said guide, corresponding ends of said strips being displaced longitudinally in said guide by a distance equal to a quarter wave length at the frequency of said energy, and means for adjustably projecting the intermediate portions of said strips from said respective wall toward the center of said guide.

3. In a high frequency system, a hollow tubular guide for an electromagnetic wave, said guide having a, conductive inner surface, a pair of elongated flexible strips, each having a conductive surface layer on at least one side thereof with a resistance substantially greater than that of said guide surface, said strips being positioned within said guide with each strip extending lengthwise of the guide with said side substantially parallel to the electric field of said wave and each strip being secured to a wall of said guide at two points spaced lengthwise of the strip, at least one of said points comprising a slidable connection, said strips being mounted along opposite walls of said guide, and means engaging both of said strips intermediate said two points on each to effect bowing of said strips, said means being adjustable to vary simultaneously the extent of bowing of both strips and thereby vary the attenuation offered by the strips to said wave.

4. In a high frequency system, a hollow tubular guide for an electromagnetic wave, said guide having a conductive inner surface and a rectangular transverse section with a pair of wide walls and a pair of narrow walls, a pair of elongated strips, each having a conductive surface layer on at least one side thereof with a resistance substantially greater than that of said guide surface, said strips being positioned within said guide alongside opposite narrow walls thereof, each of said strips extending lengthwise of said guide with said side of the strip substantially perpendicular to said wide walls and being secured to the corresponding narrow wall at two points spaced lengthwise of the strip, the connection at one of said points being slidable, and means engaging both of said strips intermediate said two points on each to effect bowing of said strips, said means being adjustable to vary simultaneously the extent of bowing of both strips and thereby vary the amount of attenuation o1- fered thereby to said wave.

5. A variable attenuator comprising a section of wave guide of rectangular cross-section in which said guide has unequal cross-sectional dimensions, a pair of resistive vanes positioned within said guide substantially opposite each other and substantially parallel to the sides of site directions through substantially equal distances.

6. A variable attenuator comprising a section of wave guide of rectangular cross-section in which said guide has unequal cross-sectional dimensions, a pair of elongated resistive members positioned within said guide adjacent the sides of said guide having the shorter dimension, said members having portions which are coextensive longitudinally of said guide, and means for moving the central portions of said members laterally in opposite directions through substantially equal distances.

RICHARD B. NELSON.

REFERENCES orrnn' The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,197,123 King Apr. 16, 1940 

